Visual working memory for Depth is strictly limited

While visual working memory (WM) for attributes such as color and shape is limited to 4 objects, WM for metric depth (in our labs) is 1 or even less. Our subjects saw 4 items (letters or numerals), placed each in its own depth plane, for 200 or 800 ms, retained them for 700 or 900 ms, and then received a cue to report the item in a specified depth plane (partial report) or received a display with the depth of one item changed on half the trials (change detection). Eye movements (saccades or vergence movements) were discouraged by central fixation. Reports were highly inaccurate even after training. When the number of items (N) was increased, Cowan’s capacity K=N(Hits-False alarms) remained 1 or less. In contrast, reports of the same items laid out in the picture plane were accurate up to K=4 items, the usual WM limit. We propose that there exists a supplementary WM for location in peri-personal space, possibly in the dorsal stream, which is subject to what may be called a ‘motoric’ limitation; since only one item can be acted upon at a time, non-attended items in this WM are treated as distractors and suppressed from awareness.